1. Field of the Invention
The present invention relates to an apparatus for assembling a fuel assembly and more specifically relates to an apparatus for mechanically or automatically removing the keys which had been inserted into the supporting grids of a fuel assembly for deflecting the springs away from the dimples to enable smooth loading of the fuel rods into the supporting grids.
2. Background Technology
In general, pressurized light water nuclear reactors utilize a type of fuel assembly comprising: an upper nozzle and a lower nozzle; a plurality of supporting grids for supporting the fuel rods (shortened to grids henceforth) disposed between the nozzles with a certain spacing; instrumentation pipes and fuel rod guide pipes inserted into and attached to the grid cells of the grids; and a plurality of fuel rods inserted into and held elastically in the grid cells between the springs and the dimples formed inside the grid cells.
As shown in FIGS. 3 and 4, the grid 1 is made by intersecting a plurality of straps 1a which cross each other at right angles. As shown in FIG. 3, the arrangement of the dimples 2 and springs 3 is symmetrical with respect to an axis at right angles to the paper passing through the center O formed by the intersection of a line X--X and a line Y--Y. The dimples 2 and the springs 3 are disposed so that they are symmetrical in each of the four quadrants. In FIG. 3, the reference numeral 4 and 5 designate, respectively, grid cells 7 for housing a control rod guide pipe and housing instrumentation pipes. In FIG. 4, the reference numeral 6 refers to an inner key.
When assembling such a fuel assembly, there is a danger of introducing scratches on the surface of the fuel rods if the rods are inserted into the grid cells with the springs and ,dimples protruding out into the grid cells, because the springs and dimples are formed on the inside walls of the grid cells. To prevent damaging the surface of the fuel rods, there have been various methods suggested to deal with the prevention of surface damage problem.
For example, a method proposed in a U.S. Pat. No. 5,068,081 (Japanese Patent Application No. S64-957) proposed the following fuel assembly. In this fuel assembly, insertion openings 8 are formed in the grids 1, and the spring deflection jigs (not shown) are inserted into the grid cells 7 through the insertion openings 8 to deflect the springs 3 away from the dimples 2 formed in each wall of the grid cell 7 for elastically holding the fuel rod. The keys 6 are inserted through the key insertion openings 8 to keep the springs 3 in the deflected position, so as to allow the fuel rods to be inserted into the grid cells 7 smoothly without the danger of causing surface damage. To remove the keys 6 from the fuel assembly, the keys are first moved along the direction of the key axis, and the hook 6a is disengaged from the rib 1b of the strap 1a. Next, the keys 6 are rotated about the key axis through 90 degrees thereby orienting the keys 6 so that the hook 6a would not interfere with the rim section 8a of the insertion openings 8, and the keys 6 are removed from the grid 1 by withdrawing the keys 6 along the direction of the key axis 6.
Further, an improvement in the key design such as the one shown in FIG. 6 has also been made. The key 9 shown in FIG. 6 has a protrusion sections 9a disposed at regular intervals so as to contact the rim section 8a of the insertion openings 8 to enable to deactivate the spring 3. The protrusion section 9'a has a tapered section 9'b so that it can be moved in the key axis direction via the tapered section 9'b to disengage the protrusion section 9'a from the rim section 8a of the insertion openings 8. Next, the key 9 is rotated 90 degrees about the key axis so that the protrusion section 9'a will not interfere with the rim section 8a of the insertion openings 8, and the key 9 is removed from the grid 1.
However, such insertion and removal operations for the keys 6, 9 are based on manual approach, causing operational problems because the tasks are cumbersome, the operations 143 lengthy and the efficiency is low. Furthermore, the workers involved in the key removal operation are required to work in the proximity of the fuel rods, and are particularly vulnerable to radiation exposure. From these viewpoints, there has been a need to automate the key removal operation.